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The usual suspects of garden havoc

August 22, 2004

Spider mites


Spider mites - tiny, eight-legged animals - appear as specks on the bottoms of leaves. They require large populations to cause serious damage to plants, but their populations build up quickly when temperatures exceed 80 degrees. Plant leaves turn yellow, get stippled, develop webbing, and might drop off after mites pierce the plant's epidermal cells and extract plant sap.

Control: "Knock 'em off with a stream of water. They love things hot and dry," Mayer said. The spread of mites also might be slowed by removing damaged leaves. Spot-treat with a chemical labeled for mites when white stippling along veins on underside of leaves is first noticed and 20 mites per leaflet are present.

Tomato hornworm


These caterpillars are green with white V-shaped lines on the sides, up to 4 inches long with a posterior horn. They prefer to eat tomato leaves - and may burrow into fruit - but they also attack eggplant, pepper and potato plants.

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Control: Hornworms can easily be hand-picked from plants and squashed or drowned. Hornworms and other vegetable-munching caterpillars also can be controlled with neem oil, Sabadilla or Bacillus thuringiensis (Bt), which is safe for humans and animals. A parasitic wasp called the braconid also destroys tomato hornworms. Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate. The wasps will kill the hornworms when they emerge from the cocoons.

Colorado potato beetle


This hard-shelled, round beetle with black and yellow stripes running lengthwise on its shell prefers to strip the foliage off potato plants, but the insects also will eat cabbage, pepper, tomato, eggplant and other popular garden goodies.

Control: Straw mulch will keep Colorado potato beetles from emerging from the soil, Mayer said. Create a garden habitat that attracts such potato beetle predators as toads, birds, stink bugs and ground beetles, she said. Covering plants with lightweight insect barriers can keep beetles off plants, and alternating potato and bean plants within each row confuses the offending bugs, Mayer said.

"It messes with their minds," she said.

Squash bug


Squash bugs - soft-shelled insects that are about 1/2 inch long - suck the juices from cucumbers, gourds, melons, squash and other vine plants.

Control: When possible, plant insect-resistant varieties of these vine plants. Tachinid flies also attack squash bugs, so attract these beneficial insects with dill, fennel, yarrow and Queen Anne's lace.

Mexican bean beetle


Adult Mexican bean beetles - which are 1/3-inch-long, hard-bodied insects that are round or oval and yellow to coppery brown in color with 16 black spots - remove leaf tissue between the veins of leaves. Females lay clusters of yellow eggs on the bottoms of leaves.

Control: Planting potatoes between each bean plant might confuse the beetles, and soy beans "work well as a trap crop for bean beetles," Mayer said. She also suggested planting goldenrod, Queen Anne's lace and wild daisies in or near the garden to attract such natural Mexican bean beetle enemies as soldier bugs and assassin bugs. The beetles can be hand-picked from plants, but insecticides might be necessary if defoliation exceeds 20 percent before plants bloom or 10 percent during podding.

Cucumber beetle


Cucumber beetles - which are about 1/4 inch long with a black head and yellow thorax that's either striped or spotted - often invade in large numbers, and can transmit a bacteria that causes infected plants to wilt and defoliate. The beetles suck juices from cucumbers and related squash crops.

Control: Row covers are effective against the beetles, but the insect shields must be removed during flowering to allow for pollination. Cucumber beetles can be hand-picked from plants and drowned in insecticidal soap or dish-washing liquid, Mayer said. She also suggested planting dill, fennel, yarrow and Queen Anne's lace to attract tachinid flies, which combat cucumber beetles.

Onion thrips


Onion thrips - adults of which are 1/25 inch long, slender and light yellow to brown in color - puncture the outer layer of the onion's leaves and feed on sap and bits of leaf tissue, leaving small whitish blotches on the leaves. Red onions are most susceptible to thrip damage. Thrips thrive in hot, dry weather.

Control: Thrips are hard to control because they feed between leaves, but it helps to regularly hose down plants early in the day when injury is first noticed. Rotating onion crops each year also can help, Mayer said. Insecticides targeted at onion thrips and applied with enough water to ensure coverage is the most effective control measure, but resistance to insecticides is a problem.

Slugs


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